The invention relates to a method of producing oxidic, antireflective coatings on transparent plastic substrates, especially on sheets and lenses of polydiethyleneglycoldiallylcarbonate, in which the substrate is vacuum coated with an initial layer of high index of refraction and then with a second layer of lower index of refraction.
The demand for stable antireflective coatings on transparent substrates has long existed. The fact that they improve transmission by about 8 to 10% is of secondary importance. What is more important is the elimination of light reflection, which is especially desirable when the substrate in question is situated between an observer or viewer and an object. This is particularly the case, for example, with aircraft and motor vehicle windshields and with eyeglass lenses. Especially in the dark, light rays which strike the substrate from in back of the viewer produce reflections which are several times brighter than the observed object and virtually blind the observer. Sunlight striking the glazing of a helicopter cockpit from the rear, or light from the headlights of an automobile reflected from the windshield of a car in front of it, are examples of this situation. But even light coming from the direction of the object can produce undesirable reflections when, after passing through the substrate, it is reflected from behind it, for example by light-colored clothing or by the cornea of the human eye. The latter circumstance is a particular nuisance in the case of eyeglass lenses.
While the antireflective coating of inorganic substrates has been accomplished in a substantially satisfactory manner, the situation is still very poor in regard to plastic substrates. The reason for this is to be found in the fact that, on the basis of physical laws, the index of refraction of the antireflective coating must be lower than the index of refraction of the substrate. Consequently, a large number of oxidic materials are unusable for the production of antireflective coatings. As for the rest of them, some are difficult to vaporize, and with others the substrate has to be heated to temperatures between 300.degree. and 350.degree. C., either during the vacuum coating or in an after-treatment, in order to achieve the desired coating properties. Such substrate temperatures, however, are impractical for obvious reasons. It is not possible, for example, to use on plastic substrates the magnesium fluoride which can be used on inorganic substrates. Microfissures form in the coating; the coating comes off even in the sweat test. In addition, a single-layer coating does not produce sufficient antireflective effect. The reasons for this are familiar to the average technically trained person. The efforts are therefore all directed towards multiple layers or laminated systems.
In the book, "Die Fachvortrage des WVAO-Jahreskongresses 1973 in Berlin," published by the Wissenschaftliche Vereinigung fur Augenoptik und Optometrie e.V., of Bad Godesberg, it is recommended in connection with the antireflective coating of plastic glasses and in consideration of the above-mentioned physical laws regarding the refractive indices, that first the plastic substrate be coated with a layer of higher refractive index of about n=2, and then with a silicate layer. The following are given as requirements of the coating:
(a) Low reflection at the maximum sensitivity of the eye for light, PA1 (b) Freedom from absorption, PA1 (c) Strength of adhesion, PA1 (d) Hardness PA1 (e) Resistance to chemicals, sweat and mold, PA1 (f) Resistance to wiping (scratch resistance), PA1 (g) Temperature stability, PA1 (h) Low aging effect.
It has been found that, with the plastic glass coatings thus specified, a temperature stability up to about 90.degree. to 100.degree. C. is obtained, but only in the dry state, not, for example, in the sweat test.
For example, the attempt has been made to improve the teaching given in the above citation by applying, as the first, highly refractive layer, one of titanium dioxide, and, as the second, less refractive layer, one of magnesium fluoride. It has been found, however, that the second layer is easily removed during the sweat test and upon heating, while the first layer usually remains undamaged. The latter by itself, however, does not fulfill the purpose of a high reduction of reflection. With regard to the index of refraction, cryolite might serve for the second layer, but its hygroscopicity precludes its use.
It has furthermore been found that plastics show an extraordinarily different behavior under vacuum coating conditions. This is the case, for example, with regard to surface properties before and after any glow discharge treatment that may be necessary, and with regard to the removal of monomers by evaporation, and consequently also with regard to strength of adhesion. Aging during the later use of the vacuum coated objects differs completely from one plastic to another. Finally, there are great differences in the index of refraction, which is what determines the optical properties in conjunction with thin coatings, and the thermal expansion coefficient and modulus of elasticity, which are important to mechanical strength. For each plastic and for each group of plastics, it is therefore necessary to develop a tailored-to-measure vacuum coating procedure, especially when, for other reasons, the plastic must comply with other special requirements which have no connection with the coating, such as resistance to shattering, for example.